Compact zoom lens system

ABSTRACT

A zoom lens system comprising a first lens group having positive refractive power, a second lens group having negative refractive power and a third lens group having positive refractive power, the zoom lens system being arranged to be zoomed from the wide position to the teleposition by keeping the second lens group fixed in respect to the image surface and moving the first and third lens groups respectively toward the direction away from the image surface and arranged to be focused on an object at the infinite distance up to an object at a short distance by moving the first lens group only toward the direction away from the image surface, the zoom lens system having a large zoom ratio and being compact in size.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a compact zoom lens system for stillcameras having a large zoom ratio from the wide position to theteleposition.

(b) Description of the Prior Art

The range of focal length most widely adopted for interchangeable lenssystems for still cameras is 35 mm to 100 mm when converted to 35 mmformat film size. Therefore, there ia a high demand for a zoom lenssystem which covers the above-mentioned range of focal lengths. In otherwords, there is a high demand in the market for a compact zoom lenssystem having the zoom ratio of 2.8 and range of field angle 2ω=62°˜24°and arranged to be small in size and light in weight.

This kind of known zoom lens systems are disclosed in Japanese publishedunexamined patent application Nos. 30855/79, 156912/80 and 48607/81.However, these zoom lens systems are not compact enough because thediameter of front lens is large and overall length is somewhat long.

SUMMARY OF THE INVENTION

it is, therefore, a primary object of the present invention to provide acompact zoom lens system for still cameras with three-group lensconfiguration for which the zoom ratio is 2.8 and range of field angleis 24°˜62°.

The zoom lens system according to the present invention has the basicarrangement as shown in FIG. 1 and comprises a first lens group I havingpositive power, a second lens group II having negative power and a thirdlens group III having positive power. Besides, the zoom lens systemaccording to the present invention is arranged that, at the time ofzooming from the wide position to the teleposition, the first lens groupI and third lens group III are moved toward the object side and thesecond lens group II is kept fixed in respect to the image surface. Asthe above-mentioned zooming method to move the first and third lensgroups is adopted, it is possible to simplify the lens tube. Besides, asthe second lens group which causes the largest unfavourable influence ofeccentricity is arranged to be kept fixed in respect to the imagesurface, adjustment of lenses at the time of assembly is much simplifiedand it is very advantageous for the manufacture.

As the above-mentioned lens-group moving method is adopted, the zoomlens system according to the present invention enables to make theoverall length of the lens system in the wide position short comparedwith known lens systems for example, the zoom lens system according toJapanese published unexamined patent application No. 48607/81) in whichthe first surface in the lens system is kept fixed in respect to theimage surface at the time of zooming and, moreover, the degree offreedom at the time of the design increases. Besides, the zoom lenssystem according to the present invention is arranged to move the firstlens group toward the object side when focusing the lens system on anobject at a short distance from the state that it is focused on anobject at the infinite distance.

The zooms lens system according to the present invention ischaracterized in that the above-mentioned zooming method is adopted asthe basic arrangement and, moreover, the aperture stop is arranged to bemoved, at the time of zooming, between the second lens group and thirdlens group independently of the movements of lens groups.

With high vari-focal ratio zoom lens systems, widths of variation ofmagnification to be alloted to respective lens groups are large.Therefore, in both of the wide position and teleposition, there existsuch lens groups for which the distance between the principal points ofthose lens groups becomes large and, consequently, the states of raysbecome largely different in individual positions of zooming (wideposition˜teleposition) when the magnification is varied. For example, incase of a lens system in which the stop is provided just behind thesecond lens group, the front lens may have a small diameter as far asthe offaxial rays in the wide position are concerned. However, in thethird lens group, those rays pass through the marginal portions ofrespective lenses constituting the third lens group and, as a result, itbecomes very difficult to well balance the quality of image formed inthe central portion of image with the quality of image formed byoffaxial rays. On the other hand, in case of a lens system in which thestop is provided just in front of the third lens group, the diameter ofthe front lens should be made very large. Besides, in the wide position,offaxial rays pass through the marginal portion of the second lens groupand, therefore, it becomes difficult to correct aberrations.

Due to the reason described in the above, it is effective for improvingthe performance of the lens system when the position of the stop isselected so that the offaxial rays pass through respective lenses atheights most favourable for correction of aberrations when the lenssystem is in the wide position.

Besides, when the aperture stop is arranged to be moved according to themovements of lens groups for zooming, it is possible to prevent the lensdiameters from becoming large and it is very effective for balancing ofaberrations.

The zoom lens system according to the present invention is arranged thatthe aperture stop is located in a position approximately in the middlebetween the second lens group and the third lens groups and, at the sametime, it is so arranged that the aperture stop is moved at the time ofzooming, independently of the movements of respective lens groups, inthe same direction as the moving direction of the first lens group andthe third lens group by a moving amount smaller than the moving amountsof the first and third lens groups (about 1/2 of the moving amounts ofthe first and third lens groups) so that the aperture stop always comesduring zooming to a position approximately in the middle between thesecond and third lens groups.

The focal length f of a lens system having composition similar to thezoom lens system according to the present invention can be expressed bythe formula (i) shown below where reference symbol f₁ represents thefocal length of the first lens group and reference symbols β₂ and β₃respectively represent magnifications of the second and third lensgroups.

    f=f.sub.1 ·β.sub.2 ·β.sub.3    (i)

Besides, the distance H₁₂ between the principal points of the first andsecond lens groups can be expressed by the formula (ii) shown belowwhere reference symbol f₂ represents the focal length of the second lensgroup.

    H.sub.12 =f.sub.1 +f.sub.2 (1-1/β.sub.2)              (ii)

In the same way as above, the distance H₂₃ between the principal pointsof the second and third lens groups can be expressed by the formula(iii) shown below where reference symbol f₃ represents the focal lengthof the third lens group.

    H.sub.23 =f.sub.2 (1-β.sub.2)+f.sub.3 (1-1/β.sub.3) (iii)

When, for the distances H₁₂ and H₂₃ between the principal points andmagnification β₂ and β₃, their values in the wide position are expressedby attaching the suffix W and their values in the teleposition areexpressed by attaching the suffix T, the amount of variation ΔH₁₂ of thedistance H₁₂ between the principal points of the first and second lensgroups when the lens system is zoomed from the wide position to theteleposition is expressed by the formula (iv) shown below.

    ΔH.sub.12 =H.sub.12T -H.sub.12W =f.sub.2 (1/β.sub.2W -1/β.sub.2T)                                         (iv)

In the same way as above, the amount of variation ΔH₂₃ of the distanceH₂₃ between the principal points of the second and third lens groups isexpressed by the formula (v) shown below.

    ΔH.sub.23 =H.sub.23W -H.sub.23T =f.sub.2 (β.sub.2T -β.sub.2W)+f.sub.3 (1/β.sub.3T -1/β.sub.3W) (v)

To make the lens system compact (to make the overall length of the lenssystem short) and to minimize the variations of aberrations to be causedat the time of zooming, it is perferable to make ΔH₁₂ and ΔH₂₃ small.

However, due to the reason described below, it is not possible to makeΔH₁₂ and ΔH₂₃ too small.

In the formula (iv), ΔH₁₂ can be made small when f₂ is made small or(1/β_(2W) -1/β_(2T)) is made small. When it is tried to make ΔH₁₂ smallby making (1/β_(2W) -1/β_(2T)) small, the zoom ratio becomes smallbecause the difference between β_(2W) and β_(2T) is made small withoutchanging the lenses. This conflicts with the basic requirement of a highvari-focal ratio zoom lens system (requirement to make the zoom ratio aslarge as possible).

When ΔH₁₂ is made small by making f₂ small without changing the value of(1/β_(2W) -1/β_(2T)), the zoom ratio does not become small, and this ispreferable. However, the power of the second lens group becomes strongand it becomes considerably difficult to correct aberrations caused bythe second lens group. To prevent the above-mentioned aggravation ofaberrations caused by the second lens group, it may be considered toincrease the number of lenses constituting the second lens group. If,however, the number of lenses is increased, airspaces in respect toadjacent lens groups become small and it becomes impossible to move thelens groups largely. As a result, it becomes impossible to make the zoomratio large.

Matters described in the above are applicable also for ΔH₂₃. That is,ΔH₂₃, f₂, f₃, β_(2T), β_(2W), β_(3T) and β_(3W) have the samecorrelation as described in the above.

Therefore, to materialize a zoom lens system which enables to attain theobject of the present invention, i.e., a zoom lens system which hascomparatively high vari-focal ratio and is arranged that the lens systemas a whole is compact and aberrations are corrected favourably, it isvery important to well balance the focal lengths of respective lensgroups and ranges of magnifications of the second and third lens groups,which constitute the zooming subsystem.

From the viewpoints described so far, for the zoom lens system accordingto the present invention composed as explained in the above, it iseffective when said zoom lens system is arranged to fulfill thefollowing conditions where reference symbol f_(W) represents the focallength of the lens system as a whole in the wide position.

(1) 1.5<f₁ /f_(W) <2.5

(2) -0.65<f₂ /f_(W) <-0.39

(3) -1.6<1/β_(2W) -1/β_(2T) <-0.7 1.8<β_(2T) /β_(2W) <2.2

(4) 0.75<f₃ /f_(W) <1.1

(5) 0.2<1/β_(3T) -1/β_(3W) <0.3 1.2<β_(3T) /β_(3W) <1.6

The condition (1) defines the focal length of the first lens group. Thiscondition serves in combination with the conditions (3) and (5) toobtain a compact zoom lens system which covers the range of field angle2ω=62°˜24° (focal length in the range of 36 mm to 101 mm when convertedto 35 mm format film size). If, in the condition (1), f₁ /f_(W) becomeslarger than the upper limit or smaller than the lower limit, aberrationsbecome unfavourable. Besides, the amounts of variation of magnificationsβ₂ and β₃ necessarily become large, and the balance of distances betweenlens groups described below becomes unfavourable. That is, in case of azoom lens system composed liked the zoom lens system according to thepresent invention, the distance between the first lens group I and thesecond lens group II becomes short in the wide position and the distancebetween the second lens group II and the third lens group III becomesshort in the teleposition. When the first lens group I and the thirdlens group III are moved as described in the above, it is perferable toarrange that the positions of said lens groups after movement becomeapproximately the same as the position obtained by reversing thepositions of said lens groups before movement round the fixed lens group(the second lens group II). In other words, it is perferable to arrangethat the first and third lens groups are moved in such manner that, forexample, the distance between the first and second lens groups in thewide position becomes approximately the same as the distance between thesecond and third lens groups in the teleposition because this manner oflens group movements is favourable for correction of aberrations andbalancing of powers of respective lens groups. Therefore, theabove-mentioned lens group moving method is called the moving method inwhich the distances between lens groups are well balanced.

The condition (2) relates to the condition (3) and defines the range ofΔH₁₂ expressed by the formula (iv). If f₂ /f_(w) becomes smaller thanthe lower limit of the condition (2), ΔH₁₂ becomes large. If, on thecontrary, f₂ /f_(W) becomes larger than the upper limit of the condition(2), power of the second lens group becomes too strong and aberrationscaused by the second lens group become large.

The condition (3) relates to the condition (2) as it is evident from theformula (iv). If the values defined by the condition (3) becomes smallerthan the lower limits thereof, ΔH₁₂ becomes large and it becomesimpossible to make the lens system compact. If, on the contrary, thevalues defined by the condition (3) become larger than the upper limitsthereof, it becomes impossible to make the zoom ratio large though ΔH₁₂becomes small.

The condition (4) relates to the condition (5) and defines the distancebetween the second lens group and the third lens group. If f₃ /f_(W)becomes smaller than the lower limit of the condition (4), power of thethird lens group becomes too strong and it is not preferable forcorrection of aberrations. Besides, the balance of distances betweenlens groups in the afore-mentioned meaning becomes unsatisfactory. If,on the contrary, f₃ /f_(W) becomes larger than the upper limit of thecondition (4), the value of ΔH₂₃ becomes large and, moreover, thedistance from the lens system to the image surface becomes too large.Therefore, it becomes impossible to make the lens system compact.

The condition (5) is established based on the relation between thecondition (4) and formula (v). If the values defined by the condition(5) become larger than the upper limits thereof, it becomes impossibleto make the lens system compact. Besides, as the offaxial rays passthrough the marginal posrtion of the third lens group, aberrations areaggravated. If, on the contrary, the values defined by the condition (5)become smaller than the lower limits thereof, it becomes impossible tomake the zoom ratio large.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the basic arrangement of the zoom lens system according tothe present invention;

FIG. 2 shows a sectional view of the zoom lens system according to thepresent invention;

FIGS. 3 through 5 respectively show graphs illustrating aberrationcurves of Embodiment 1 of the present invention at f=36.23, f=60.5 andf=101.3;

FIGS. 6 through 8 respectively show graphs illustrating aberrationcurves of Embodiment 2 of the present invention at f=36.22, f=60.5 andf=101.3; and

FIGS. 9 through 11 respectively show graphs illustrating aberrationcurves of Embodiment 3 of the present invention at f=36.22, f=64.35 andf=101.35.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, preferred embodiments of the zoom lens system according to thepresent invention are described below.

As shown in FIG. 2 illustrating the sectional view of the zoom lenssystem according to the present invention, preferred embodiments of thezoom lens system according to the present invention are arranged thatthe first lens group I comprises a cemented doublet L₁, L₂ and apositive meniscus lens L₃, the second lens group II comprises a negativemeniscus lens L₄, a cemented doublet L₅, L₆ and a cemented doublet L₇,L₈, and the third lens group III comprises a positive lens L₉, apositive lens L₁₀, a positive lens L₁₁ (these three positive lenses maybe arranged as two positive lenses), a negative meniscus lens L₁₂arranged to be concave toward the object side, a negative meniscuc lensL₁₃ arranged to be convex toward the object side, a positive lens L₁₄and a positive lens L₁₅. By adopting the above-mentioned lensconfiguration, the zoom lens system according to the present inventionis arranged to well balance aberrations.

In the above-mentioned lens configuration, the third lens group isarranged to have the basic composition of triplet type. However, thenumber of lens elements constituting respective lens components in thetriplet type composition are increased in order to increase theaberration correcting capacity. Out of these lens components, thenegative lens component is important and it is arranged to comprise thenegative meniscus lens L₁₂ which is concave toward the object side andthe negative meniscus lens L₁₃ which is convex toward the object side.Out of them, the lens L₁₂ is arranged to have a surface with strongnegative power on the object side (surface r₂₁) in order to maintainsymmetry of offaxial rays. Besides, to correct overcorrected sphericalaberration which is caused by the second lens group and theabove-mentioned surface with strong negative power (r₂₁), it is soarranged that an air lens with strong positive power is formed betweenthe lens L₁₂ and lens L₁₃. Furthermore, for these negative lenselements, the radius of curvature R of the surface on the image side ofthe lens L₁₃ (r₂₄ in FIG. 2) and total thickness D of the lenses L₁₂ andL₁₃ (d₂₁ +d₂₃ in FIG. 2) are selected in the ranges which fulfill thefollowing conditions.

(6) 0.7<R/f₃ <1.2

(7) 0.25<D/f₃ <0.35

In the condition (6), R/f₃ is defined to be smaller than the upper limitof 1.2 in order to arrange that the principal point of the third lensgroup comes to a position shifted from the surface r₁₅ toward the imageside by an amount about 1/4 of the thickness of the third lens group andto thereby keep H₂₃ in the wide position as large as possible. Besides,R/f₃ is defined to be larger than the lower limit of 0.7 in order tocorrect spherical aberration and coma favourably.

The condition (7) defines the range of D for the purpose of correctingastigmatic difference caused by the first and second lens groups. IfD/f₃ becomes larger than the upper limit of the condition (7), itbecomes difficult to limit offaxial aberrations and distortion bykeeping the lens system compact. If D/f₃ becomes smaller than the lowerlimit of the condition (7), it becomes difficult to keep astigmaticdifference as a small value.

Now preferred embodiments of the zoom lens system according to thepresent invention are shown below.

    ______________________________________                                        Embodiment 1                                                                  ______________________________________                                        r.sub.1 = 136.988                                                             d.sub.1 = 2.5  n.sub.1 = 1.80518                                                                           ν.sub.1 = 25.43                               r.sub.2 = 53.184                                                              d.sub.2 = 9    n.sub.2 = 1.51633                                                                           ν.sub.2 = 64.15                               r.sub.3 = -104.043                                                            d.sub.3 = 0.1                                                                 r.sub.4 = 30.479                                                              d.sub.4 = 4.8  n.sub.3 = 1.65160                                                                           ν.sub.3 = 58.67                               r.sub.5 = 56.486                                                              d.sub.5 = l.sub.1                                                                            (0.764˜9.901˜17.377)                               r.sub.6 = 52.644                                                              d.sub.6 = 1.5  n.sub.4 = 1.77250                                                                           ν.sub.4 = 49.66                               r.sub.7 = 16.297                                                              d.sub.7 = 3.2                                                                 r.sub.8 = 152.848                                                             d.sub.8 = 3    n.sub.5 = 1.72825                                                                           ν.sub.5 = 28.46                               r.sub.9 = -40.125                                                             d.sub.9 = 1    n.sub.6 =  1.72                                                                             ν.sub.6 = 50.25                               r.sub.10 = 76.052                                                             d.sub.10 = 2.8                                                                r.sub.11 = -17.423                                                            d.sub.11 = 1   n.sub.7 = 1.66671                                                                           ν.sub.7 = 57.33                               r.sub.12 = 27.345                                                             d.sub.12 = 3.2 n.sub.8 = 1.83400                                                                           ν.sub.8 = 37.16                               r.sub.13 = -55.989                                                            d.sub.13 = l.sub.21                                                                          (7.673˜2.376˜0.548)                                r.sub.14 = ∞ (stop)                                                     d.sub.14 = l.sub.22                                                                          (10.418˜7.184˜0.293)                               r.sub.15 = 31.182                                                             d.sub.15 = 5.14                                                                              n.sub.9 = 1.48749                                                                           ν.sub.9 = 70.15                               r.sub.16 = -80.213                                                            d.sub.16 = 2                                                                  r.sub.17 = 957.849                                                            d.sub.17 = 3.3 n.sub.10 = 1.48749                                                                          ν.sub.10 = 70.15                              r.sub.18 = -61.412                                                            d.sub.18 = 0.11                                                               r.sub.19 = 100.245                                                            d.sub.19 = 3   n.sub. 11 = 1.617                                                                           ν.sub.11 = 62.79                              r.sub.20 = -131.354                                                           d.sub.20 = 4.25                                                               r.sub.21 = -22.878                                                            d.sub.21 = 2   n.sub.12 = 1.84666                                                                          ν.sub.12 = 23.88                              r.sub.22 = -35.645                                                            d.sub.22 = 0.2                                                                r.sub.23 = 68.089                                                             d.sub.23 = 8.05                                                                              n.sub.13 = 1.80518                                                                          ν.sub.13 = 25.43                              r.sub.24 = 30.546                                                             d.sub.24 = 3.47                                                               r.sub.25 = 15585.232                                                          d.sub.25 = 4   n.sub.14 = 1.61921                                                                          ν.sub.14 = 60.27                              r.sub.26 = -45.091                                                            d.sub.26 = 0.11                                                               r.sub.27 = 51.892                                                             d.sub.27 = 5.08                                                                              n.sub.15 = 1.55960                                                                          ν.sub.15 = 58.63                              r.sub.28 = 133.727                                                            f = 36.235˜101.295, F 4.1                                               f.sub.1 = 62.433, f.sub.1 /f.sub.W = 1.717                                    f.sub.2 = -17.786, f.sub. 2 /f.sub.W = -0.491                                 f.sub.3 = 32.417, f.sub.3 /f.sub.W = 0.895                                    β.sub.2.sbsb.W = -0.5091, β.sub.2.sbsb.S = -0.6894,                 β.sub.2.sbsb.T = -0.9707                                                 β.sub.3.sbsb.W = -1.1401, β.sub.3.sbsb.S = -1.4033,                 β.sub.3.sbsb.T = -1.6723                                                 1/β.sub.2 W - 1/β.sub.2 T = -0.994, β.sub.2 T/β.sub.2     W = 1.907                                                                     1/β.sub.3.sbsb.T - 1/β.sub.3.sbsb.W = 0.279, β.sub.3.sbsb.T     /β.sub.3.sbsb.W = 1.467                                                 r.sub.24 /f.sub.3 = 0.942, (d.sub.21 + d.sub.23)/f.sub.3                      ______________________________________                                        = 0.31                                                                    

    ______________________________________                                        Embodiment 2                                                                  ______________________________________                                        r.sub.1 = 325.502                                                             d.sub.1 = 2.5  n.sub.1 = 1.81379                                                                           ν.sub.1 = 35.4                                r.sub.2 = 50.724                                                              d.sub.2 = 9    n.sub.2 = 1.51633                                                                           ν.sub.2 = 68.96                               r.sub.3 = -85.321                                                             d.sub.3 = 0.1                                                                 r.sub.4 = 35.934                                                              d.sub.4 = 5.2  n.sub.3 = 1.834                                                                             ν.sub.3 = 40.62                               r.sub.5 = 56.505                                                              d.sub.5 = l.sub.1                                                                            (1.531˜16.459˜27.537)                              r.sub.6 = 65.784                                                              d.sub.6 = 1.5  n.sub.4 = 1.834                                                                             ν.sub.4 = 47.72                               r.sub.7 = 16.446                                                              d.sub.7 = 3.2                                                                 r.sub.8 = -93.764                                                             d.sub.8 = 3.6  n.sub.5 = 1.48749                                                                           ν.sub.5 = 46.35                               r.sub.9 = -44.124                                                             d.sub.9 = 1    n.sub.6 =  1.83481                                                                          ν.sub.6 = 46.83                               r.sub.10 = -77.685                                                            d.sub.10 = 2.8                                                                r.sub.11 = -18.935                                                            d.sub.11 = 1   n.sub.7 = 1.66259                                                                           ν.sub.7 = 61.89                               r.sub.12 = 27.100                                                             d.sub.12 = 3   n.sub.8 = 1.83481                                                                           ν.sub.8 = 36.61                               r.sub.13 = -45.231                                                            d.sub.13 = l.sub.21                                                                          (6.82˜1.522˜0.25)                                  r.sub.14 = ∞ (stop)                                                     d.sub.14 = l.sub.22                                                                          (10.418˜7.707˜0.6)                                 r.sub.15 = 28.716                                                             d.sub.15 = 5.14                                                                              n.sub.9 = 1.48749                                                                           ν.sub.9 = 70.15                               r.sub.16 = -161.540                                                           d.sub.16 = 2.3                                                                r.sub.17 = 558.626                                                            d.sub.17 = 3.3 n.sub.10 = 1.56873                                                                          ν.sub.10 = 68.78                              r.sub.18 = -64.622                                                            d.sub.18 = 0.11                                                               r.sub.19 = 92.049                                                             d.sub.19 =  3  n.sub.11 = 1.63306                                                                          ν.sub.11 = 63.26                              r.sub.20 = -137.918                                                           d.sub.20 = 4.25                                                               r.sub.21 = -24.097                                                            d.sub.21 = 2   n.sub.12 = 1.82889                                                                          ν.sub.12 = 20.34                              r.sub.22 = -42.366                                                            d.sub.22 = 0.2                                                                r.sub.23 = 100.979                                                            d.sub.23 = 8.16                                                                              n.sub.13 = 1.83641                                                                          ν.sub.13 = 31.52                              r.sub.24 = 30.921                                                             d.sub.24 = 3.47                                                               r.sub.25 = 345.629                                                            d.sub.25 = 4   n.sub.14 = 1.62026                                                                          ν.sub.14 = 59.37                              r.sub.26 = -56.495                                                            d.sub.26 = 0.1                                                                r.sub.27 = 63.594                                                             d.sub.27 = 5.08                                                                              n.sub.15 = 1.56873                                                                          ν.sub.15 = 57.74                              r.sub.28 = -291.508                                                           f = 36.22˜101.307, F 4.1                                                f.sub.1 = 80.018, f.sub.1 /f.sub.W = 2.209                                    f.sub.2 = -20.157, f.sub.2 /f.sub.W = -0.557                                  f.sub.3 = 35.169, f.sub.3 /f.sub.W = 0.971                                    β.sub.2.sbsb.W = -0.3876, β.sub.2.sbsb.S = -0.5436,                 β.sub.2.sbsb.T = -0.7752                                                 β.sub.3.sbsb.W = -1.1678, β.sub.3.sbsb.S = -1.3929,                 β.sub.3.sbsb.T = -1.6339                                                 1/β.sub.2.sbsb.W - 1/β.sub.2.sbsb.T = -1.29, β.sub.2.sbsb.T     /β.sub.2.sbsb.W = 2                                                     1/β.sub.3.sbsb.T - 1/β.sub.3.sbsb.W = 0.244, β.sub.3.sbsb.T     /β.sub.3.sbsb.W = 1.399                                                 r.sub.24 /f.sub.3 = 0.896, (d.sub.21 + d.sub.23)/f.sub.3                      ______________________________________                                        = 0.295                                                                   

    ______________________________________                                        Embodiment 3                                                                  ______________________________________                                        r.sub.1 = 148.501                                                             d.sub.1 = 2.5  n.sub.1 = 1.80518                                                                           ν.sub.1 = 25.43                               r.sub.2 = 58.233                                                              d.sub.2 = 9    n.sub.2 = 1.51633                                                                           ν.sub.2 = 64.15                               r.sub.3 = -106.930                                                            d.sub.3 = 0.1                                                                 r.sub.4 = 31.261                                                              d.sub.4 = 5.2  n.sub.3 = 1.72916                                                                           ν.sub.3 = 54.68                               r.sub.5 = 56.152                                                              d.sub.5 = l.sub.1                                                                            (0.55˜10.49˜16.62)                                 r.sub.6 = 53.077                                                              d.sub.6 = 1.5  n.sub.4 = 1.7725                                                                            ν.sub.4 = 49.66                               r.sub.7 = 15.712                                                              d.sub.7 = 3.2                                                                 r.sub.8 = 139.937                                                             d.sub.8 = 3.4  n.sub.5 = 1.60342                                                                           ν.sub.5 = 38.01                               r.sub.9 = -29.521                                                             d.sub.9 = 1    n.sub.6 = 1.804                                                                             ν.sub.6 = 46.57                               r.sub.10 = 207.838                                                            d.sub.10 = 2.8                                                                r.sub.11 = -18.409                                                            d.sub.11 = 1   n.sub.7 = 1.66687                                                                           ν.sub.7 = 57.33                               r.sub.12 = 28.284                                                             d.sub.12 = 3.2 n.sub.8 = 1.834                                                                             ν.sub.8 = 37.16                               r.sub.13 = -54.062                                                            d.sub.13 = l.sub.21                                                                          (7.4˜1.402˜0.274)                                  r.sub.14 = ∞ (stop)                                                     d.sub.14 = l.sub.22                                                                          (10.42˜7˜0.4)                                      r.sub.15 = 31.123                                                             d.sub.15 = 5.14                                                                              n.sub.9 = 1.48749                                                                           ν.sub.9 = 70.15                               r.sub.16 = -76.631                                                            d.sub.16 = 2.02                                                               r.sub.17 = 1490.2                                                             d.sub.17 = 3.3 n.sub.10 = 1.48749                                                                          ν.sub.10 = 70.15                              r.sub.18 = -60.161                                                            d.sub.18 = 0.1                                                                r.sub.19 = 109.458                                                            d.sub.19 = 3   n.sub.11 = 1.617                                                                            ν.sub.11 = 62.79                              r.sub.20 = -140.632                                                           d.sub.20 = 4.25                                                               r.sub.21 = -22.961                                                            d.sub.21 = 2   n.sub.12 = 1.84666                                                                          ν.sub.12 = 23.88                              r.sub.22 = -35.748                                                            d.sub.22 = 0.2                                                                r.sub.23 = 67.841                                                             d.sub.23 = 8.15                                                                              n.sub.13 = 1.80518                                                                          ν.sub.13 = 25.43                              r.sub.24 = 30.118                                                             d.sub.24 = 3.47                                                               r.sub.25 = -1582.82                                                           d.sub.25 = 4   n.sub.14 = 1.61941                                                                          ν.sub.14 = 60.27                              r.sub.26 = -43.736                                                            d.sub.26 = 0.11                                                               r.sub.27 = 45.619                                                             d.sub.27 = 5.08                                                                              n.sub.15 = 1.56101                                                                          ν.sub.15 = 58.63                              r.sub.28 = 108.887                                                            f = 36.22˜101.35, F 4.1                                                 f.sub.1 = 60.207, f.sub.1 /f.sub.W = 1.662                                    f.sub.2 = -17.331, f.sub. 2 /f.sub.W = -0.478                                 f.sub.3 = 32.637, f.sub.3 /f.sub.W = 0.901                                    β.sub.2 W = -0.5204, β.sub.2 S = -0.7419, β.sub.2 T =          -1.0057                                                                       β.sub.3.sbsb.W = -1.156, β.sub.3 S = -1.440, β.sub.3 T =       -1.674                                                                        1/β.sub.2 W - 1/β.sub.2 T = -0.927, β.sub.2 T/β.sub.2     W = 1.933                                                                     1/β.sub.3.sbsb.T - 1/β.sub.3.sbsb.W = 0.268, β.sub.3.sbsb.T     /β.sub.3.sbsb.W = 1.448                                                 r.sub.24 /f.sub.3 = 0.923, (d.sub.21 + d.sub.23)/f.sub.3                      ______________________________________                                        = 0.31                                                                    

In embodiments shown in the above, reference symbols r₁ through r₂₈respectively represent radii of curvature of respective lens surfaces,reference symbols d₁ through d₂₇ respectively represent thicknesses ofrespective lenses and airspaces between respective lenses, referencesymbols n₁ through n₁₅ respectively represent refractive indices ofrespective lenses, reference symbols ν₁ through ν₁₅ respectivelyrepresent Abbe's numbers of respective lenses, reference symbol f_(W)represents the focal length of the lens system as a whole in the wideposition, reference symbols β_(2W), β_(2S) and β_(2T) respectivelyrepresent maginifications of the second lens group in the wide position,standard position and teleposition, and reference symbols β_(3W), β_(3S)and β_(3T) respectively represent magnifications of the third lens groupin the wide position, standard position and teleposition.

In respective embodiments shown in the above, the second lens group iskept fixed in respect to the image surface and the first lens group, thethird lens group and the stop are moved. Therefore, airspaces d₅ =l₁,d₁₃ =l₂ and d₁₄ =l₃ vary (d₁₃ represents the airspace between the secondlens group and the stop, and d₁₄ represents the airspace between thestop and the third lens group). Values of these variable airspaces inthe wide position, standard position and teleposition are shown inparentheses in the numerical data of respective embodiments.

I claim:
 1. A zoom lens systemconsisting of, in order from the objectside, a first lens group located in the foremost position and havingpositive refractive power, a second lens group located in a positionsubsequent to said first lens group and having negative refractivepower, and a third lens group located subsequent to said second lensgroup and having positive refractive power, said zoom lens system beingzoomed from a wide position to a teleposition by keeping said secondlens group fixed with respect to an image surface, and moving said firstlens group and said third lens group respectively toward the directionaway from the image surface independently of each other by individuallydifferent amounts of movements, said zoom lens system being focused onan object at an infinite distance up to an object at a short distance bymoving only said first lens group toward the direction away from theimage surface.
 2. A zoom lens system according to claim 1, furthercomprising:an aperture stop located between said second lens group andsaid third lens group, said stop arranged to be moved during zoomingindependently of the movements of said respective lens groups.
 3. A zoomlens system according to claim 2, whereinsaid aperture stop is moved toa position approximately in the middle of said second lens group andsaid third lens group.
 4. A zoom lens system according to claim 3,whereinsaid zoom lens system is arranged to fulfill the followingconditions:(1) 1.5<f₁ f_(W) <2.5 (2) -0.65<f₂ /f_(W) <-0.39 (3)-1.6<1/β_(2W) -1/β_(2T) <-0.7 1.8<β_(2T) /β_(2W) <2.2 (4) 0.75<f₃ /f_(W)<1.1 (5) 0.2<1/β_(3T) -1/β_(3W) <0.3 1.2<β_(3T) /β_(3W) <1.6 wherein,reference symbol f_(W) represents the focal length of the lens system asa whole in the wide position, reference symbols f₁, f₂ and f₃respectively represent focal lengths of the first, second and third lensgroups, reference symbols β_(2T) and β_(2W) respectively representmagnifications of the second lens group in the teleposition and wideposition, and reference symbols β_(3T) and β_(3W) respectively representmagnifications of the third lens group in the teleposition and wideposition.
 5. A zoom lens system according to claim 4, whereinsaid thirdlens group at least comprises a negative meniscus lens arranged to beconcave toward the object side and a negative meniscus lens arranged tobe convex toward the object side and located adjacent to and on theimage side of said negative meniscus lens concave toward the objectside, and wherein said zoom lens system is arranged to further fulfillthe following conditions:(6) 0.7<R/f₃ <1.2 (7) 0.25<D/f₃ <0.35 wherein,reference symbol R represents the radius of curvature of the surface onthe image side of said negative meniscus lens arranged to be convextoward the object side, and reference symbol D represents the totalthickness of said two negative meniscus lenses.
 6. A zoom lens systemaccording to claim 5, whereinsaid first lens group comprises a cementeddoublet and a positive meniscus lens, said second lens group comprises anegative meniscus lens and two cemented doublets, and said third lensgroup comprises three positive lenses, a negative meniscus lens arrangedto be concave toward the object side, a negative meniscus lens arrangedto be convex toward the object side, and two positive lenses, said zoomlens system having the following numerical data:

    ______________________________________                                        r.sub.1 = 136.988                                                             d.sub.1 = 2.5  n.sub.1 = 1.80518                                                                           ν.sub.1 = 25.43                               r.sub.2 = 53.184                                                              d.sub.2 = 9    n.sub.2 = 1.51633                                                                           ν.sub.2 = 64.15                               r.sub.3 = -104.043                                                            d.sub.3 = 0.1                                                                 r.sub.4 = 30.479                                                              d.sub.4 = 4.8  n.sub.3 = 1.65160                                                                           ν.sub.3 = 58.67                               r.sub.5 = 56.486                                                              d.sub.5 = l.sub.1                                                                            (0.764˜9.901˜17.377)                               r.sub.6 = 52.644                                                              d.sub.6 = 1.5  n.sub.4 = 1.77250                                                                           ν.sub.4 = 49.66                               r.sub.7 = 16.297                                                              d.sub.7 = 3.2                                                                 r.sub.8 = 152.848                                                             d.sub.8 = 3    n.sub.5 = 1.72825                                                                           ν.sub.5 = 28.46                               r.sub.9 = -40.125                                                             d.sub.9 = 1    n.sub.6 = 1.72                                                                              ν.sub.6 = 50.25                               r.sub.10 = 76.052                                                             d.sub.10 = 2.8                                                                r.sub.11 = -17.423                                                            d.sub.11 = 1   n.sub.7 = 1.66671                                                                           ν.sub.7 = 57.33                               r.sub.12 = 27.345                                                             d.sub.12 = 3.2 n.sub.8 = 1.83400                                                                           ν.sub.8 = 37.16                               r.sub.13 = -55.989                                                            d.sub.13 = l.sub.21                                                                          (7.673˜2.376˜0.548)                                r.sub.14 = ∞ (stop)                                                     d.sub.14 = l.sub.22                                                                          (10.418˜7.184˜0.293)                               r.sub.15 = 31.182                                                             d.sub.15 = 5.14                                                                              n.sub.9 = 1.48749                                                                           ν.sub.9 = 70.15                               r.sub.16 = -80.213                                                            d.sub.16 = 2                                                                  r.sub.17 = 957.849                                                            d.sub.17 = 3.3 n.sub.10 = 1.48749                                                                          ν.sub.10 = 70.15                              r.sub.18 = -61.412                                                            d.sub.18 = 0.11                                                               r.sub.19 = 100.245                                                            d.sub.19 = 3   n.sub.11 =  1.617                                                                           ν.sub.11 = 62.79                              r.sub.20 = -131.354                                                           d.sub.20 = 4.25                                                               r.sub.21 = -22.878                                                            d.sub.21 = 2   n.sub.12 = 1.84666                                                                          ν.sub.12 = 23.88                              r.sub.22 = -35.645                                                            d.sub.22 = 0.2                                                                r.sub.23 = 68.089                                                             d.sub.23 = 8.05                                                                              n.sub.13 = 1.80518                                                                          ν.sub.13 = 25.43                              r.sub.24 = 30.546                                                             d.sub.24 = 3.47                                                               r.sub.25 = 15585.232                                                          d.sub.25 = 4   n.sub.14 = 1.61921                                                                          ν.sub.14 = 60.27                              r.sub.26 = -45.091                                                            d.sub.26 = 0.11                                                               r.sub.27 = 51.892                                                             d.sub.27 = 5.08                                                                              n.sub.15 = 1.55960                                                                          ν.sub.15 = 58.63                              r.sub.28 = 133.727                                                            f = 36.235˜101.295, F 4.1                                               f.sub.1 = 62.433, f.sub.1 /f.sub.W = 1.717                                    f.sub.2 = -17.786, f.sub.2 /f.sub.W  = -0.491                                 f.sub.3 = 32.417, f.sub.3 /f.sub.W = 0.895                                    β.sub.2 W = -0.5091, β.sub.2 S = -0.6894, β.sub.2 T =          -0.9707                                                                       β.sub.3 W = -1.1401, β.sub.3 S = -1.4033, β.sub.3 T =          -1.6723                                                                       1/β.sub.2 W - 1/β.sub.2 T = -0.994, β.sub.2 T/β.sub.2     W = 1.907                                                                     1/β.sub.3 T - 1/β.sub.3 W = 0.279, β.sub.3 T/β.sub.3      W = 1.467                                                                     r.sub.24 /f.sub.3 = 0.942, (d.sub.21 + d.sub.23)/f.sub.3                      ______________________________________                                        = 0.31                                                                    

wherein reference symbols r₁ through r₂₈ respectively represent radii ofcurvature of respective lens surfaces, reference symbols d₁ through d₂₇respectively represent thicknesses of respective lenses and airspacesbetween respective lenses, reference symbols n₁ through n₁₅ respectivelyrepresent refractive indices of respective lenses, reference symbols ν₁through ν₁₅ respectively represent Abbe's numbers of respective lenses,and reference symbols β₂ S and β₃ S respectively representmagnifications of the second and third lens groups in the standardposition.
 7. A zoom lens system according to claim 5, whereinsaid firstlens group comprises a cemented doublet and a positive meniscus lens,said second lens group comprises a negative meniscus lens and twocemented doublets, and said third lens group comprises three positivelenses, a negative meniscus lens arranged to be concave toward theobject side, a negative meniscus lens arranged to be convex toward theobject side, and two positive lenses, said zoom lens system having thefollowing numerical data:

    ______________________________________                                        r.sub.1 = 325.502                                                             d.sub.1 = 2.5  n.sub.1 = 1.81379                                                                           ν.sub.1 = 35.4                                r.sub.2 = 50.724                                                              d.sub.2 = 9    n.sub.2 = 1.51633                                                                           ν.sub.2 = 68.96                               r.sub.3 = -85.321                                                             d.sub.3 = 0.1                                                                 r.sub.4 = 35.934                                                              d.sub.4 = 5.2  n.sub.3 = 1.834                                                                             ν.sub.3 = 40.62                               r.sub.5 = 56.505                                                              d.sub.5 = l.sub.1                                                                            (1.531˜16.459˜27.537)                              r.sub.6 = 65.784                                                              d.sub.6 = 1.5  n.sub.4 = 1.834                                                                             ν.sub.4 = 47.72                               r.sub.7 = 16.446                                                              d.sub.7 = 3.2                                                                 r.sub.8 = -93.764                                                             d.sub.8 = 3.6  n.sub.5 = 1.48749                                                                           ν.sub.5 = 46.35                               r.sub.9 = -44.124                                                             d.sub.9 = 1    n.sub.6 = 1.83481                                                                           ν.sub. 6 = 46.83                              r.sub.10 = -77.685                                                            d.sub.10 = 2.8                                                                r.sub.11 = -18.935                                                            d.sub.11 = 1   n.sub.7 = 1.66259                                                                           ν.sub.7 = 61.89                               r.sub.12 = 27.100                                                             d.sub.12 = 3   n.sub.8 = 1.83481                                                                           ν.sub.8 = 36.61                               r.sub.13 = -45.231                                                            d.sub.13 = l.sub.21                                                                          (6.82˜1.522˜0.25)                                  r.sub.14 = ∞ (stop)                                                     d.sub.14 = l.sub.22                                                                          (10.418˜7.707˜0.6)                                 r.sub.15 = 28.716                                                             d.sub.15 = 5.14                                                                              n.sub.9 = 1.48749                                                                           ν.sub.9 = 70.15                               r.sub.16 = -161.540                                                           d.sub.16 = 2.3                                                                r.sub.17 = 558.626                                                            d.sub.17 = 3.3 n.sub.10 = 1.56873                                                                          ν.sub.10 = 68.78                              r.sub.18 = -64.622                                                            d.sub.18 = 0.11                                                               r.sub.19 = 92.049                                                             d.sub.19 = 3   n.sub.11 =  1.63306                                                                         ν.sub.11 = 63.26                              r.sub.20 = -137.918                                                           d.sub.20 = 4.25                                                               r.sub.21 = -24.097                                                            d.sub.21 = 2   n.sub.12 = 1.82889                                                                          ν.sub.12 = 20.34                              r.sub.22 = -42.366                                                            d.sub.22 = 0.2                                                                r.sub.23 = 100.979                                                            d.sub.23 = 8.16                                                                              n.sub.13 = 1.83641                                                                          ν.sub.13 = 31.52                              r.sub.24 = 30.921                                                             d.sub.24 = 3.47                                                               r.sub.25 = 345.629                                                            d.sub.25 = 4   n.sub.14 = 1.62026                                                                          ν.sub.14 = 59.37                              r.sub.26 = -56.495                                                            d.sub.26 = 0.1                                                                r.sub.27 = 63.594                                                             d.sub.27 = 5.08                                                                              n.sub.15 =  1.56873                                                                         ν.sub.15 = 57.74                              r.sub.28 = -291.508                                                           f = 36.22˜101.307, F 4.1                                                f.sub.1 = 80.018, f.sub.1 /f.sub.W = 2.209                                    f.sub.2 = -20.157, f.sub.2 /f.sub.W = -0.557                                  f.sub.3 = 35.169, f.sub.3 /f.sub.W = 0.971                                    β.sub.2 W = -0.3876, β.sub.2 S = -0.5436, β.sub.2 T =          -0.7752                                                                       β.sub.3 W = -1.1678, β.sub.3 S = -1.3929, β.sub.3 T =          -1.6339                                                                       1/β.sub.2 W - 1/β.sub.2 T = -1.29, β.sub.2 T/β.sub.2      W = 2                                                                         1/β.sub.3 T - 1/β.sub.3 W = 0.244, β.sub.3 T/β.sub.3      W = 1.399                                                                     r.sub.24 /f.sub.3 = 0.896, (d.sub.21 + d.sub.23)/f.sub.3                      ______________________________________                                        = 0.295                                                                   

wherein reference symbols r₁ through r₂₈ respectively represent radii ofcurvature of respective lens surfaces, reference symbols d₁ through d₂₇respectively represent thicknesses of respective lenses and airspacesbetween respective lenses, reference symbols n₁ through n₁₅ respectivelyrepresent refractive indices of respective lenses, reference symbols ν₁through ν₁₅ respectively represent Abbe's numbers of respective lenses,and reference symbols β₂ S and β₃ S respectively representmagnifications of the second and third lens groups in the standardposition.
 8. A zoom lens system according to claim 5, whereinsaid firstlens group comprises a cemented doublet and a positive meniscus lens,said second lens group comprises a negative meniscus lens and twocemented doublets, and said third lens group comprises three positivelenses, a negative meniscus lens arranged to be concave toward theobject side, a negative meniscus lens arranged to be convex toward theobject side, and two positive lenses, said zoom lens system having thefollowing numerical data:

    ______________________________________                                        r.sub.1 = 148.501                                                             d.sub.1 = 2.5  n.sub.1 = 1.80518                                                                           ν.sub.1 = 25.43                               r.sub.2 = 58.233                                                              d.sub.2 = 9    n.sub.2 = 1.51633                                                                           ν.sub.2 = 64.15                               r.sub.3 = -106.930                                                            d.sub.3 = 0.1                                                                 r.sub.4 = 31.261                                                              d.sub.4 = 5.2  n.sub.3 = 1.72916                                                                           ν.sub.3 = 54.68                               r.sub.5 = 56.152                                                              d.sub.5 = l.sub.1                                                                            (0.55˜10.49˜16.62)                                 r.sub.6 = 53.077                                                              d.sub.6 = 1.5  n.sub.4 = 1.7725                                                                            ν.sub.4 = 49.66                               r.sub.7 = 15.712                                                              d.sub.7 = 3.2                                                                 r.sub.8 = 139.937                                                             d.sub.8 = 3.4  n.sub.5 = 1.60342                                                                           ν.sub.5 = 38.01                               r.sub.9 = -29.521                                                             d.sub.9 = 1    n.sub.6 = 1.804                                                                             ν.sub.6 = 46.57                               r.sub.10 = 207.838                                                            d.sub.10 = 2.8                                                                r.sub.11 = -18.409                                                            d.sub.11 = 1   n.sub.7 = 1.66687                                                                           ν.sub.7 = 57.33                               r.sub.12 = 28.284                                                             d.sub.12 = 3.2 n.sub.8 = 1.834                                                                             ν.sub.8 = 37.16                               r.sub.13 = -54.062                                                            d.sub.13 = l.sub.21                                                                          (7.4˜1.402˜0.274)                                  r.sub.14 = ∞ (stop)                                                     d.sub.14 = l.sub.22                                                                          (10.42˜7˜0.4)                                      r.sub.15 = 31.123                                                             d.sub.15 = 5.14                                                                              n.sub.9 = 1.48749                                                                           ν.sub.9 = 70.15                               r.sub.16 = -76.631                                                            d.sub.16 = 2.02                                                               r.sub.17 = 1490.2                                                             d.sub.17 = 3.3 n.sub.10 = 1.48749                                                                          ν.sub.10 = 70.15                              r.sub.18 = -60.161                                                            d.sub.18 = 0.1                                                                r.sub.19 = 109.458                                                            d.sub.19 = 3   n.sub.11 =  1.617                                                                           ν.sub.11 = 62.79                              r.sub.20 = -140.632                                                           d.sub.20 = 4.25                                                               r.sub.21 = -22.961                                                            d.sub.21 = 2   n.sub.12 = 1.84666                                                                          ν.sub.12 = 23.88                              r.sub.22 = -35.748                                                            d.sub.22 = 0.2                                                                r.sub.23 = 67.841                                                             d.sub.23 = 8.15                                                                              n.sub.13 = 1.80518                                                                          ν.sub.13 = 25.43                              r.sub.24 = 30.118                                                             d.sub.24 = 3.47                                                               r.sub.25 = -1582.82                                                           d.sub.25 = 4   n.sub.14 = 1.61941                                                                          ν.sub.14 = 60.27                              r.sub.26 = -43.736                                                            d.sub.26 = 0.11                                                               r.sub.27 = 45.619                                                             d.sub.27 = 5.08                                                                              n.sub.15 = 1.56101                                                                          ν.sub.15 = 58.63                              r.sub.28 = 108.887                                                            f = 36.22˜101.35, F 4.1                                                 f.sub.1 = 60.207, f.sub.1 /f.sub.W = 1.662                                    f.sub.2 = -17.331, f.sub.2 /f.sub.W  = -0.478                                 f.sub.3 = 32.637, f.sub.3 /f.sub.W = 0.901                                    β.sub.2 W = -0.5204, β.sub.2 S = -0.7419, β.sub.2 T =          -1.0057                                                                       β.sub.3 W = -1.156, β.sub.3 S = -1.440, β.sub.3 T =            -1.674                                                                        1/β.sub.2 W - 1/β.sub.2 T = -0.927, β.sub.2 T/β.sub.2     W = 1.933                                                                     1/β.sub.3 T - 1/β.sub.3 W = 0.268, β.sub.3 T/β.sub.3      W = 1.448                                                                     r.sub.24 /f.sub.3 = 0.923, (d.sub.21 + d.sub.23)/f.sub.3                      ______________________________________                                        = 0.31                                                                    

wherein reference symbols r₁ through r₂₈ respectively represent radii ofcurvature of respective lens surfaces, reference symbols d₁ through d₂₇respectively represent thicknesses of respective lenses and airspacesbetween respective lenses, reference symbols n₁ through n₁₅ respectivelyrepresent refractive indices of respective lenses, reference symbols ν₁through ν₁₅ respectively represent Abbe's numbers of respective lenses,and reference symbols β₂ S and β₃ S respectively representmagnifications of the second and third lens groups in the standardposition.